There are various corrosion inhibiting compositions which are effective in cooling water systems with respect to mild steel and copper. Except for silicate salts, typical corrosion inhibitors have been characterized as environmental pollutants. For example, phosphates, nitrites, nitrates and borates are potential algae nutrients when discharged from wastewater. Algae blooms, in turn, give rise to eutrophication of water. It is also believed that phosphonates will behave similarly.
Chromate/zinc based corrosion inhibiting compositions for open cooling water corrosion inhibition have long been used commercially and are extremely effective. However, heavy metals have been identified as carcinogens and toxins. Therefore, the level of heavy metals admissible to the environment is restricted. With increased environmental awareness, corrosion inhibiting compositions based upon heavy metals are now unacceptable.
High inorganic phosphate/organic phosphate blends have been substituted for corrosion inhibiting compositions containing heavy metals, but there is now an environmental concern with phosphate discharge. This has lead to the use of lower amounts of inorganic phosphate/organic phosphates blends. Care must also be taken when using inorganic phosphate/organic phosphate blends because they can contribute fouling and/or pitting.
It is known to use silicates in potable water circuits as a corrosion inhibitor and that silicates are non toxic and will not cause stress to the environment. It is also known that silicates and their combination with other inhibitors, such as nitrate, nitrite, orthophosphate, molybdate, azoles, and borate can be used in closed automotive cooling circuits with successful mitigation of corrosion. This is because in closed cooling circuits, fresh water is not continuously added since there is no significant loss of fluid due to evaporation. Consequently, fluid impurities resulting from evaporation are not concentrated in the system.
However, in open evaporative recirculating cooling water circuits, the use of silicates has not been successful compared to chromate/zinc, inorganic phosphate blends, zinc phosphonate, and organic phosphonates. This is because in open recirculating cooling water circuits, fresh water is added to the cooling fluid due to evaporative losses, and fluid impurities are concentrated due to evaporation. Such losses from the bulk recirculating water also results in the reduction in the amount of silicate inhibitor available for corrosion inhibitory film formation and film maintenance.
Furthermore, over time, changes in the pH and temperature of the system water will promote the formation of polymeric silicates from naturally occurring monomeric silicates in the system water. These polymeric silicates decrease the functioning of the corrosion inhibiting compositions in the system water and increase the potential for foulant deposition. Also, since the performance of silicate based corrosion inhibitors against pitting and fouling are known to be pH sensitive and temperature sensitive, it is believed that they may promote the formation of magnesium silicate-on the metal surfaces which come into contact with the circulating water. Because of these problems, there is skepticism about using silicate based corrosion inhibiting compositions in open cooling systems.